Electrostatic comb drive devices are utilized to provide movement or motion in microelectromechanical systems (MEMS) devices. Such drive devices are employed, for example, in the fabrication of MEMS-type accelerometers, gyroscopes, and inertia sensing devices where rapid actuation is often necessary to effectively measure and/or detect motion and acceleration.
In a typical comb drive device, a main body is supported over an underlying support substrate using a number of anchors. One or more drive elements electrically coupled to the main body can be actuated to manipulate the main body above the support substrate in a particular manner. In certain designs, for example, the drive elements may include a number of interdigitated comb fingers configured to convert electrical energy into mechanical energy using electrostatic actuation.
One method of fabrication of electrostatic comb drive devices generally begins with a silicon wafer substrate. A highly boron-doped layer is realized through diffusion or epitaxial growth over the wafer substrate, which can then be etched to form the desired microstructures using a patterned mask layer and a suitable etch process, such as the Bosch-type Deep Reactive Ion Etch (DRIE). The etched wafer is then bonded to an underlying support substrate using a suitable bonding process such as anodic bonding. The support substrate may include a number of mesas that support the main body and drive elements above the support substrate while allowing movement thereon, and metal patterns appropriate for connecting to the silicon members. One or more electrodes can also be provided on the support substrate to measure up/down movement of the main body caused by, for example, acceleration or rotation of the sensor. The silicon substrate wafer is then removed through one or more non-selective and selective etch processes, such as KOH and EDP based etching, leaving only the patterned, highly doped silicon structure.
For the force of the comb drive to be applied uniformly as the device moves back and forth, the shape or profile of the etched structure should be as uniform as possible. The uniformity of the etched structure is dependent on a number of factors including, for example, the gap between adjacent features, and the parameters of the DRIE process used. Since etching tends to be slower at those locations where there are relatively small gaps between adjacent features, the profile of the comb fingers tend to be non-uniform along their length, due to the varying gap sizes caused by the partial overlap of the comb fingers. This non-uniformity may result in changes in capacitance as the comb fingers move with respect to each other, producing undesired electrical harmonics in the motor drive power. These additional harmonics can reduce the desired motive force of the comb fingers, resulting in greater energy dissipation and noise in the sensor output. In some cases, the non-linear profile of the comb fingers may produce quadrature, or motion out-of-plane, which further creates noise in the sensor output. As such, there is a need in the art for improved fabrication methods for reducing harmonic distortion in comb drive devices.